Recently, the trend toward global environmental protection has resulted in emission gas standards for automobiles being tightened and weight saving of automobile bodies for improved mileage being promoted. In a recently demanded method for weight saving of automobile bodies, solid-core parts using bar steel are replaced with hollow ones using steel tubes. Stabilizers, which prevent automobile bodies from rolling when cornering thereby stabilizing high-speed driving, are no exception of this trend; solid-core products using bar steel are being changed to hollow ones using steel tubes (hollow stabilizers) for the purpose of weight saving of automobile bodies.
Such hollow stabilizers are usually produced in the following way: materials, namely, seamless steel tubes or electric resistance welded steel tubes, are cold-formed into a desired shape and then subjected to thermal refining including quenching or quenching and tempering. Among other materials, electric resistance welded steel tubes are relatively inexpensive and excellent in terms of accuracy of dimensions and thus are widely used as materials for hollow stabilizers. For example, Japanese Examined Patent Application Publication No. H1-58264 proposes a steel for electric resistance welded steel tubes for hollow stabilizers; this steel contains C: 0.35% or less, Si: 0.25% or less, Mn: 0.30 to 1.20%, Cr: less than 0.50%, N+O: 0.0200% or less, Ti: from four to twelve times the content of (N+O) contained in the steel, and B: 0.0005 to 0.009% and may further contain Ca: 200 ppm or less and/or Nb: at most 4/10 of the content ratio of C, wherein the content ratio of C, Si, Mn, and Cr is controlled so that DI will be 1.0 in. or more and that Ceq will be 0.60% or less.
Japanese Examined Patent Application Publication No. S61-45688 proposes a method for manufacturing a steel for electric resistance welded steel tubes for hollow stabilizers; in this method, a steel slab containing C: 0.35% or less, Si: 0.25% or less, Mn: 0.30 to 1.20%, Cr: less than 0.50%, N+O: 0.0200% or less, Ti: from four to twelve times the content of (N+O) contained in the steel, and B: 0.0005 to 0.009% and possibly further containing Ca: 200 ppm or less, wherein the content ratio of C, Si, Mn, and Cr is controlled so that DI will be 1.0 in. or more and that Ceq will be 0.60% or less, is hot-rolled and then coiled with the coiling temperature maintained in the range of 570 to 690° C.
Also, Japanese Unexamined Patent Application Publication No. H6-93339 proposes a method for manufacturing a strong and highly ductile electric resistance welded steel tube for stabilizers or the like. The technology described in Japanese Unexamined Patent Application Publication No. H6-93339 is a method for manufacturing a strong and highly ductile electric resistance welded steel tube in which a material, steel containing C: 0.18 to 0.28%, Si: 0.10 to 0.50%, Mn: 0.60 to 1.80%, Ti: 0.020 to 0.050%, and B: 0.0005 to 0.0050%, possibly further containing at least one of Cr: 0.20 to 0.50%, Mo: 0.5% or less, and Nb: 0.015 to 0.050%, and possibly further containing Ca: 0.0050% or less, is processed into an electric resistance welded steel tube, and then the electric resistance welded steel tube is subjected to a normalizing treatment at a temperature in the range of 850 to 950° C. followed by quenching.
For the relatively low cost and excellent accuracy of dimension thereof, electric resistance welded steel tubes have been widely used as materials for hollow stabilizers. However, recent attempts to reduce more weight of hollow stabilizers have forced them to be exposed to greater stresses; as a result, the arts described in Japanese Examined Patent Application Publication Nos. H1-58264 and S61-45688 and Japanese Unexamined Patent Application Publication No. H6-93339 fail to ensure sufficient durability, in particular, the durability of electric resistance welded portions, by themselves. This situation is attributable to insufficient hardenability of the electric resistance welded portions. More specifically, the hardness after quenching of electric resistance welded portions drops when electric resistance welded steel tubes are cold-bent into a desired shape and then quenched by rapid electric heating for a short time, sometimes leading to the decreased durability of a resultant material.
It could therefore be helpful to provide an electric resistance welded steel tube that is excellent in terms of flatness, has a sustained hardness after quenching of electric resistance welded portions even after being subjected to rapid quenching which is used for hollow stabilizers or the like, and thus can be used as a material that is excellent in terms of durability as well as a method for manufacturing such an electric resistance welded steel tube.